Process for the preparation of melamine



March 11, 1969 F. WElNROTTER ET 3,432,501

- PROCESS FOR THE PREPARATION OF MELAMINE Filed April 4, 1967 INVENTORS FERDINAND WElNROTTER ALFRED SQHMIDT WALTER M I. JLLER WALTER BOHLER BY LUW a! a fi j ATTORNEYS 3,432,501 PROCESS FOR THE PREPARATION OF MELAMINE Ferdinand Weinrotter and Alfred Schmidt, Linz, Walter Miiller, Leonding, near Linz, and Walter Biihler, Linz, Austria, assignors to Osterreichische Stickstotfwerke Aktiengesellschaft, Linz (Danube), Austria Filed Apr. 4, 1967, Ser. No. 628,434 Claims priority, application Austria, Apr. 5, 1966, A 3,218/66 US. Cl. 260249.7 Int. Cl. C07d 55/28 4 Claims ABSTRACT OF THE DISCLOSURE Various methods have recently been described for the preparation of melamine, according to which urea or its thermal decomposition products such as cyanic acid, cyanuric acid, biuret, triuret, amn-ieline or ammelide are converted into melamine in the presence of added ammonia. If such synthesis is carried out at atmospheric pressure or slightly above atmospheric pressure, a catalyst is required if technically useful yields are to be realized.

Heretofore, the literature has described two fundamentally different types of catalysts: amorphous substances with a large interior or inner surface and crystalline substances with practically no interior surface.

Amorphous substances with an interior surface of 180 to 650 square meters per gram are known; these comprise, for example, silica gel, alumina gel, titanium, oxide gel, zirconium oxide gel, kieselguhr, pumice, etc. These substances all have in common the amorphous structure and the high internal surface. The yields of melamine achievable with these catalysts varies between 10 and 92% of the theoretical yield obtainable according to the reaction scheme:

depending upon the reaction conditions and the particular catalysts employed.

Other catalysts with no especially large internal surface are for example boron phosphate and aluminum phosphate. The yields of melamine realizable with these are up to 90%.

In addition, there are mixed catalysts, consisting of oxides of aluminum, titanium, silicon, zirconium, vanadium, chromium, iron or of carbon in admixture with an oxygen compound of phosphorus, boron, sulfur or arsenic. These substances have no internal surface. Yields of melamine achievable therewith are up to 90% of the theoretical.

Finally, catalysts have been described which consist of an aluminum oxide carrier with phosphorus-containing compound, sulfur-containing compound, boron-containmg compound or arsenic-containing compound sup- United States Patent 0 3,432,501 Patented Mar. 11, 1969 ported thereon, melamine yields obtainable therewith being up to 90.8% of the theoretical.

Examples of use of the above-enumerated catalysts, however, involve an operating period of only several hours or maximally one day. However, the technical utility of a catalyst on the basis of such data is out of the question, since a life of at least several months with undiminished or substantially undiminished activity is essential. All published data with respect to yields must therefore be evaluated and compared with caution, since valid conclusions as to the true value of a catalyst can only be drawn from the results of long-lasting tests (prolonged operating periods).

None of the heretofore-known classes of catalysts has shown satisfactory behavior in technical operation. The amorphous, large-surface substances retain a considerable amount of melamine because of their high adsorptive capacity, so that the retained melamine lingers on the hot catalyst surface before it sublimes off with the ammonia gas-carrier stream. Since, at the employed reaction temperature (350-470 C.), the melamine is no longer fully stable, a small portion thereof is continuously condensed to higher, no-longer-sublimable substances, so that in time the catalyst becomes inactive. Moreover, such large-surface catalysts are very sensitive substances which lose their catalytic property even when only slight changes-such as are unavoidable in technical operation-occur in the synthesis conditions. It is also known that amorphous silica gel tends to crystallize at high temperatures and this tendency is catalyzed by even traces of ammonia. As a result of such crystallization, the silica gel granules lose their coherence and disintegrate into dust form. This means that in the melamine synthesis, the product is always contaminated with catalyst dust and that after a certain period of operation the catalyst layer becomes clogged with the dust.

According to the invention, use is made of a substance which, surprisingly, offers the advantages of both known classes of catalysts and is free of the disadvantages thereof. This substance is crystalline aluminum oxide-particularly 'y-aluminum oxide-which, in contrast to the large surfaced alumina gels, has an inner surface of only to m. /g. Crystalline aluminum oxide of this character is commercially available for other uses. It can be prepared e.g. by heating aluminum hydroxide at a rate of 1 to 10 C. per minute to 400500 C. in the presence of water vapor at superatmospheric pressure (1.5-10 ata).

As a result of the crystalline structure of crystalline aluminum oxide, this catalystparticularly the 'Y- a a is significantly more stable and thus essentially insensitive to irregularities in operation.

The advantages of the catalysts according to the invention in comparison with the prior known catalysts will be evident from the following comparative test data:

The test apparatus employed is shown diagrammatically in the accompanying figure of drawing.

Two identical glass contact tubes 1 and 111, each having a clear inner cross section of 12.5 square centimeters, are mounted side by side in respective corresponding bores provided in an aluminum block furnace 2. The latter is electrically heated with the aid of per se conventional electrical resistance heaters 3, a constant temperature of 380 to 385 C. being automatically maintained. Conventional temperature measuring means 9 are provided. The heaters are arranged around the block 2 in a housing 4 encompassing the block. Accurately controllable screw conveyor feeding members 5 and 5a feed synthesis of melamine,.while theactivity of the contact with small inner surface remains essentially constant.

The other catalysts enumerated in Table 1 (which follows) were subjected to comparative testing under the same conditions and for a period of 25 days of uninterprilled urea of a grain size of 2 millimeters supplied 5 rupted operation. The results set forth in Table 2 (infra) from the supply containers and 10a into the upper confirm that contacts with small inner surfaces retain ends of the respective tubes 1 and 1a. The quantity of their activity for a longer period of time. urea is in each case 25 grams per hour. Similarly, 42 TABLE 1 CATALYSTS EMPLOYED normal liters of ammonia per hour are supplied at the 10 upper end of each contact tube 1 and 1a via conduits 11 N O t 1 t St t and 11a. Aluminum tubes 6 and 6a are arranged in the 0 a ays rue um intermediate part of the glass contact tubes 1 and 1a to 650 provide a progressively restricted portion 12 and 12a, I 570 respectively, after the manner shown in the drawing, thus 15 o 22g defining a conversion chamber thereabove in each tube fi Q15 555}; j 5 1 and 1a, in which chambers are supplied solid urea is fi fg -g gf g gg g converted into cyanic acid and ammonia. The result is a I, ao IIIIIIII 1 .III .II: 50 cyanic acid-ammonia mixture with a cyanic acid content of 15% by volume. This gas mixture then passes TABLE Z COMPARATIVE TEST RESULTS to and through the respective catalyst layers 7 and 7a positioned below the restricted portions 6 and 6a. The ,;,,;f catalyst consists of granules of 2 to 3 millimeters did op ameter. The catalyst volume in each contact tube amounts 94, 3 l g9 8 to 200 milliliters, the height of each catalyst layer being 25 16.5 centimeters. 38:1 The hot reaction gases leaving the catalysts contain ammonia, carbon dioxide, unreacted cyanic acid and 32:3 13%} gaseous melamine and move down to the respective re- 90x3 ceivers -8 and 8a which are filled with water, as illustrated. 1 Average over 90 days rati The reaction gases are cooled to ambient temperature in the said receivers, whereupon all the melamine sep- Whlle slhca f .hlgh mammal Fonverslonhtha arates out as a fine crystalline suspension and is deteraverage 1gmficantly less smce there 15 a mined gravimetrically as oxalate. The unreacted cyanic Strong decrease m actmty' acid gas is dissolved in the water as ammonium cyanate 35 The as aioredescnbeq can be used m which is immediately rearranged to urea and, as such, any known Q Y Q synthtisls 0f filamlne. An example remains dissolved in the water. Ammonia and carbon of Such apphcatlon for Instance the followmg: dioxide remain in the aqueous melamine suspension until EXAMPLE the latter is saturated and then pass out of the respective receivers as steam-saturated ammonia-carbon dioxide ad- 40 Startmg gas fmxture of 37 meters of cyamc mixture acid and 375 cubic meters of ammonia is passed hourly The catalyst in one of the contact tubes consists of through a Smgle loose granular P i layer of 3150 crystalline 'y-aluminum oxide with an inner surface of kllograms of grmular 'Y AI2O3 with an Internal Surface 100 m. /g., determined according to the BET-method. 9f 80 to 150 volume of such catalyst amount- The catalyst has a grain Size of about 2 mmand a Weight ing to about 3.5 cub 1c meters. The catalyst-filled chamber of 0:81 kilogram p liter. is preferably cylindrical, the circular bottom surface being The catalyst in the other contact tube is a silica gel squa-re meters m area and the helght of th? catillyst catalyst with an inner surface of 650 mg/gl determined layer being about 0.36 meter. The catalyst layer is neither extraneously heated nor extraneously cooled. The temperagcordng to E PL P i i Slze Is also ature of the cyanic acid-ammonia gas mixture upon entry a thesis, the temperature of the catalyst layer, as measured 51? iff z giih fifiiiii of by In a of starting gas mixture through the catalyst layer, there IS a ammoma are each cntact means 95% conversion. All the reaction heat liberated at the an: areas-areas;trainings 3:335,gz g gi fg as of speed of normal liters per square centimeter of contact p hot reaction gas is directly upon leaving h Sectlontempefature of h alglmmum, block thesis furnace, cooled by means of a circulating aqueous reafitlon fumaFe 1S malntamed at 380-385 dunng the melamine suspension and the thus-accumulated finely entlre test P P crystalline melamine is separated olf by means of a cen- The melamlne y f achlevedwlththe catalysts trifuge. Care must be taken in this regard that the hot y a y P6F10d 0f Operatlon 8 the followlng synthesis gas comes into contact with cooler particles li l itl l essentially the same initial capacity of the two whic'h dw fflg i iiclluid i i 0 auto ac1 a mos 0 o t e mitia amount W] e contact catalysts (melamine yields betwe n 94 and 5 iri solution as urea in the cooling liquid. To a void too felatilfe t0 the 111:6?1 p y the activity of the silica great an accumulation of urea in the said liquid, a porgit l g tg i s fiis l s g f (zigza (SP3 3 2E tiorfl of mother liquor circgliated bail; from the centrifuge e a lvl 8 1s rom time to time wit rawn. e gaseous ammonia crystalline 'yn m Oxlde with an inner surface of and carbon dioxide contained in the reaction gas mixture only m. /-g. remains essentially unimpaired. 70 is dissolved only in small amount in the aqueous mel- After 90 days of operation, the yield with the silica gel contact is only 84% while that with the 'y-aluminum oxide remains at 92%. This demonstrates that the activity of a contact with large inner surface rapidly decreases to a significant extent, in the long-continued amine suspension. The major portion leaves the suspension container as NH /CO /H O vapor mixture and can be salvaged.

The centrifuged melamine is washed free of urea and the wash water is added to the melamine suspension. The

centrifuge wet melamine is then dried. Impurities in the dried melamine are not detectable'by conventional tests. In any event, the impurities which can cause clouding lie Ammeline+ammelide: below 0.1% Melamine cyanurate: below 0.01%

(Ammeline and ammelide cyanurate) Residual moisture is below 0.01% Ash is below 0.01 Color test: below 20 APHA The determined nitrogen value of the melamine is 66.62% N. In other words, even without recrystallization, the melamine has a purity of about 99.90%.

Under the recited conditions, the catalyst retains its activity for as long as six months or more.

The conversion takes place essentially at atmospheric pressure, the only pressure employed being that required to effect passage of the initial gaseous reaction mixture uniformly through and throughout the catalyst layer.

What is claimed is:

1. In a process for the catalytic synthesis of melamine from urea or thermal decomposition products thereof at normal or slightly elevated pressure in the presence of ammonia gas, the improvement according to which the synthesis is carried out in the presence, as catalyst, of crystalline aluminum oxide with an internal surface of to square millimeters per gram.

2. The improvement according to claim 1, wherein the crystalline aluminum oxide is 'y-Al O 3. The improvement according to claim 1, wherein the starting substances are gaseous cyanic acid and ammonia.

4. The improvement according to claim 2, wherein the catalyst is 'y-Al o References Cited UNITED STATES PATENTS 2,760,96 1- 8/1956 Mackay 260-2497 3,095,416 6/1963 Crowley et al 260249.7 3,158,611 11/1964 Crowley et al. 260-249] XR 3,163,648 12/1964 Kaess et al. 260.249.7 3,290,309 12/ 1966 Marten 260-249.7 3,310,559 3/1967 Weinrotter et al. 260-2497 HENRY R. IILES, Primary Examiner. JOHN M. FORD, Assistant Examiner. 

